As a polypropylene has a superior physical property, it is used for a wide usage such as for example, a packaging film field and the like. However, as a polypropylene film has a high temperature capable of heat-sealing by a single layer and its proper range of temperature is narrow, a polypropylene random copolymer wherein ethylene and/or .alpha.-olefin is randomly copolymerized is usually used for this kind of usage in order to enhance a heat-sealing property at low temperature. Besides, a random copolymer has the good random property of comonomer, and therefore, has an excellent transparency and heat-sealing property owing to a low crystallinity and a low melting point.
On the other hand, it had a defect that a solubility to an organic solvent, for example, a saturated hydrocarbon solvent being not preferred in food sanitation increases remarkably in accordance with the increase of ethylene and/or .alpha.-olefin content.
As the process for preparing this propylene random copolymer, a try preparing a propylene random copolymer by the so-called gas phase polymerization method wherein a solvent or an active monomer liquid did not exist substantially has been done recently. However, for example, according to a method described in Japanese Unexamined Patent Publication No.63-276541(1988), a propylene-butene-1 copolymer being main component was prepared by a gas phase polymerization method. However, it had 15% or more of a soluble part in xylene at 20.degree. C. and made a blocking resistance and a bleed whitening property more worse, and therefore, it is not yet adequate as a film for packaging. Particularly, this defect is apt to appear in case of a film-forming with a big processing machine. Besides, it has a lot of low molecular weight component being not preferable for a film for packaging as a low temperature heat-sealing modifier. Therefore, although it has a good low temperature heat-sealing property, an attachment on a roll in case of orienting with a vertical roll when a film is prepared for a long time increases and a film-forming property becomes inferior.
Besides, although a method described in Japanese Unexamined Patent Publication No. 61-248740 (1986) improved a low temperature heat-sealing property as a composition, propylene-ethylene copolymer and propylene-ethylene-butene-1 copolymer being a main component have a high melting point of 140.degree. C. or more owing to a low content of comonomer and the appearance of a high hot-tack strength is yet inadequate. In Japanese Unexamined Patent Publication No.56-58861(1981), a film having an improved transparency and low temperature heat-sealing property is described, but propylene-ethylene copolymer and propylene-ethylene-butene-1 copolymer being a main component become a copolymer having a high melting point owing to a low content of comonomer and it has a problem in a hot-tack property. In Japanese Unexamined Patent Publication No.64-18633(1989), propylene-ethylene copolymer and propylene-ethylene-butene-1 copolymer being a main component are prepared by a bulk polymerization method and it is inadequate to give a preferable hot-tack property owing to a low content of comonomer.
Under these circumstances, the present inventors had already applied Japanese Patent Application No. 6-92317(1994) as a composition having an excellent balance of a low temperature heat-sealing property and a hot-tack property. Besides, the present inventors applied Japanese Patent Application No.5-262340 (1993) in order to obtain a polypropylene random copolymer composition having an excellent balance of a low temperature heat-sealing property and a hot-tack property.
The present inventors had an extensive study on the development of this polypropylene random copolymer composition. As the results, the present inventors found that the above-mentioned purpose could be attained by a composition comprising a crystalline propylene random copolymer having a specified range of .alpha.-olefin content, propylene-.alpha.-olefin random copolymer having a specified range of .alpha.-olefin content and a specified range of a crystalline propylene homopolymer, and completed the present invention.